1. Field of the Invention
This invention relates to composite blades and their manufacture and more particularly to a composite fan blade having unidirectional laminates arranged by increasing span height from the center of the blade towards the outer surfaces with taller laminates interspersed within and breaking up the height-arranged pattern.
2. Description of Related Art
A great effort is under way to replace the relatively heavy metal blades and vanes of fluid flow machines such as gas turbine engine fans and compressors with lighter composite materials. This has led to the development of composite blades and vanes having high strength, made from elongated filaments composited in a light weight matrix.
Over the years the term composite has had several meanings regarding the use of two or more materials having different properties. More recently, in the aerospace industry, the term composite has come to be defined as a material containing a reinforcement such as fibers or particles supported in a binder or matrix material. Many composites are adequate for the present invention including both metallic and non-metallic composites, however the preferred embodiment is made of a unidirectional tape material and an epoxy resin matrix. A discussion of this and other suitable materials may be found in the "Engineering Materials Handbook" by ASM INTERNATIONAL, 1987-1989 or later editions, which are incorporated herein by reference. The composite blades and airfoils of the present invention are preferably of the non-metallic type made of a material containing a fiber such as a carbonaceous, silica, metal, metal oxide, or ceramic fiber embedded in a resin material such as Epoxy, PMR15, BMI, PEEU, etc. Of particular use are fibers unidirectionally aligned into a tape that is impregnated with a resin, formed into a part shape, and cured via an autoclaving process or press molding to form a light weight, stiff, relatively homogeneous article having laminates within.
One particular problem which has discouraged the introduction of these light weight composite gas turbine engine fan blades is their particular vulnerability to what is referred to as foreign object damage (FOD). Many types of foreign objects may be entrained in the inlet of a gas turbine engine, ranging from large birds, such as sea gulls, to hailstones, sand and rain. Damage from foreign objects takes two forms. Smaller objects can erode the blade material and degrade the performance of the fan and engine. Impact by larger objects may rupture or pierce the blades. Portions of an impacted blade can be torn loose and cause extensive secondary damage to the downstream blades and other engine components.
In this regard, the consequences of foreign object damage are greatest in the low pressure compressors and fans of high bypass gas turbine engines. However, these components offer the greatest potential in weight reduction due to their large tip diameters, as great as ten feet, and spans in the order of two or more feet. Many developments have been made to prevent composite fan blade failures such as a leading edge protection strip which also helps provide erosion protection for the fan blade, and particularly for its leading edge.
One particular FOD-related failure mode of composite fan blades is bending and delamination of the blade when it is struck by a heavy object such as a bird, particularly in a region near the radially outward blade tip. This, in turn, can result in secondary engine damage as the blade fragments, including the leading edge protection strip, are ingested through the engine.
Thus, it has become highly desirable to develop light weight composite blades. Of particular importance are long span fan blades made of light weight non-metallic materials for a high bypass ratio gas turbine engines which resist delamination due to bending induced by foreign object impact into the blade.
One development to prevent delamination is disclosed in U.S. Pat. No. 4,022,547, "Composite Blade Employing Biased Layup" by Max W. Stanley, issued May 10, 1977, assigned to the present assignee, the General Electric Company, and incorporated herein by reference. Stanley discloses fabricating a fan or compressor blade by laying up and bonding together a plurality of filament laminates. The filaments of at least a portion of the laminates are skewed, in a chordwise direction, forward and aft of a non-radial blade axis, thus forming a biased lay-up with the blade center of twist biased forward or aft of the blade radial axis. This significantly increases the torsional frequency of the blade. In one embodiment, the filaments are skewed forward such that no filaments run from the blade leading edge to the blade tip but, rather, from the blade leading edge to the blade root. This orientation permits more strain produced by foreign object impact to be transmitted to the blade root where it can be more easily absorbed and dissipated by the blade supporting disc.
A typical non-metallic light weight composite airfoil lay-up manufacture provides laying up the composite airfoil as two halves, then assembling the two halves together to form the airfoil. Older composite designs have suggested the use of a single element lay-ups as described in the prior art description in U.S. Pat. No. 4,051,289, "Composite Airfoil Construction" by Arthur P. Adamson which issued Sep. 27, 1977, and is assigned to the present assignee. The lay-up procedure conventionally starts at the airfoil centerplane by laying up the two halves, convex side and concave side, separately and then bonding the two halves together. The plies are typically of varying width and span or height to form a blade of tapering thickness towards its radially outer tip.
The number of plies or laminations may run on the order of 700. Ply thickness is usually determined by the material to be used and is on the order of 5 or 6 mils. The span height, width, and shape depends, at least in part, on the shape and contour of the blade. One typical well known method of determining ply shapes and span heights provides for taking the airfoil shape laid out flat along its flattened or untwisted blade centerplane, and cutting it into plies of the desired thickness. This then determines the ply span height and shape. This often determines the ply arrangement because most ply lay-up are then arranged smallest to largest from centerplane outward.
This procedure produces a lay-up sequence that has many if not all the radially outward ply edges or tips ending at the centerplane of the airfoil. This is usually a very high stress area during an impact event or other event that causes blade bending. These conventional blade construction methods produce laminate composite blades that form shear planes along blade centerplanes where high stresses often occur. Blade delaminating shear plane stresses at fan blade centerplanes are increased by the long spans and high degrees of twist characteristic of modern high bypass ratio turbofan engines.